Ok, having done a bit of morning reading, I'm inclined to think the tax is a red herring. This is about the wall. Trump wants to get started with building it, maybe to be seen keeping his promises, maybe because it'll employ US workers. But it's expensive, so he needs some story for how it's going to be paid for, specifically how Mexico will pay for it. Even if that doesn't pan out, by the time it's clear the cost was paid by the US taxpayer, he'll have moved onto bigger things that will distract from the issue.

Alternatively, I'm wrong, and the wall really is just a huge put-on, used as leverage against e.g. the mexicans, or part of a more obscure campaign.

It doesn't benefit their employees, though, since consumer prices will rise as a result. As that happens, either the standard of living goes down, or wages go up, which raises consumer prices again.

So far, the wealth of the majority of western population has been based on cheap consumer goods. Those goods are cheap because the people making them have a lower standard of living than the people buying them. So if Trump reduces imports, that implies that to keep cheap goods, the standard of living for the portion of US population producing those goods must go down relative to the rest of the population.

Automation can alter that to a great extent, but the profits from that may just go to the owner class, not the workers.